1. Field of the Invention
The present invention generally relates to a quantum well structure and, more particularly, to a group-III nitride quantum well structure having a hexagonal shape.
2. Description of the Related Art
Group-III nitride semiconductors such as gallium nitride (GaN) and its alloys like indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN) and indium aluminum nitride (InAlN) can be used to manufacture light-emitting elements due to wide direct band gaps thereof. These light-emitting elements include light-emitting diodes (LEDs) or laser diodes (LDs), for example. The performances of the light-emitting elements can be improved if the quantum well technology is used to produce said elements, as elaborated in the cited references listed in Table 1 below:
TABLE 1The Characteristics of conventional Group-III NitrideCitedReferencesRelated Information of The Cited References1S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, Jpn. J.Appl. Phys. Lett. Part 1 34, L797 (1995)2H. Akabli, A. Almaggoussi, A. Abounadi, A. Rajira, K.Berland, T. G. Andersson, Superlattices and Microstructures52 (2012) 70-773I. Vurgaftman and J. R. Meyer, J. Appl. Phys. 94, 3675(2003)4Semiconductors: Group IV Elements and III-VCompounds(Data in Science and Technology), edited by O.Madelung (Spring, New York, 1991)5N. A. El-Masry, E. L. Piner, S. X. Liu, and S. M. Bedair,Appl. Phys. Lett. 72, 40 (1998)
It can be recognized from the above cited references that the III-nitride semiconductors can be widely used to produce high-brightness LEDs (please refer to reference 1 for details). By adjusting the indium-gallium ratio of the InxGa1-xN/GaN quantum well in the active layer of the light-emitting elements (namely, adjusting the indium content) can control the band gap of indium gallium nitride (InxGa1-xN) in the range of 0.7 eV to 3.4 eV. This range of band gap includes the wavebands of visible lights. Thus, a full-colored LED displayer can be manufactured by adjusting the waveband of the LEDs through the control of the indium content of said LEDs.
Furthermore, it can be observed from the relation chart between the band gap and the lattice constant of the group-III nitride (please refer to references 2 and 3 for details) that there is a significant lattice mismatch between gallium nitride (aGaN=0.3189 nm, cGaN=0.5185 nm) and indium nitride (aInN=0.35446 nm, cInN=0.57034 nm), as elaborated in the cited references 3 and 4. As a result, it is difficult to produce a high-quality InGaN/GaN quantum well.
However, indium atoms are volatile under high temperature (the melting point of indium is 156.6° C.), making it difficult to grow an epitaxial film of indium gallium nitride with large area and uniform distribution. Moreover, when a high-quality epitaxial layer of indium gallium nitride is grown, the epitaxial layer of indium gallium nitride is unable to contain more than 20% of indium (please refer to reference 5 for details). As a result, the wavebands of the LEDs are limited.
To solve the above problem, the commercial sapphire, silicon carbide (SiC) or silicon (Si)(111) may be used as a substrate. Based on this, the conventional plasma-assisted molecular beam epitaxy (PAMBE) mechanism is used to grow the epitaxial layer of indium gallium nitride (InxGa1-xN) under low temperature, so as to produce the light-emitting elements. However, the lattices match between the substrate and the epitaxial layer of indium gallium nitride still needs to be improved, which limits the performance of the light-emitting elements.
In conclusion, the conventional III-nitride quantum well structure has disadvantages of limited performance in addition to the limited waveband, which results in some limitations and inconveniences during the use. In light of this, it is necessary to improve the conventional quantum well structure.